Electrodialysis modules are of interest, inter alia, as components in electrodialysis stack systems for the desalination of water. The separators are positioned between an ion and cation exchange membranes in a stacked sheet arrangement and are designed to direct the orderly flow of ion-containing solutions through the stack. Because the efficiency of the stack is reduced by increased thickness of the separators, separator thickness is preferably minimized. Specifically, thin cell separators (viz., separators having thicknesses approximately 0.5 mm) are more efficient than "thick" cell separators (viz., those having thicknesses of 1.3 mm) because, among other reasons, the electrochemical resistance of the separator to current flow is reduced, the polarization effects in electrodialysis stacks can be decreased, and the thin cells are generally more economical to manufacture than thick cells.
Such thin cell separators should function in the manner of a gasket to seal the flow paths between membranes and prevent leakage of solution either between compartments within the stack or between the stack and the outside thereof. The gasket portions of the separators thus should be sufficiently resilient to conform to the irregular or rough surfaces of some types of commercial membranes and thereby form a liquid-tight seal. In addition, the separator should provide for distributing the solution into the membranes. On the other hand, because it is desirable to utilize a maximum area of the electrodialysis membranes in the desalination process, the obstruction to solution flow presented by the separator should be a minimum and hence the solution distributing and the gasket portions of the separator should cover a minimum of the total separator area.
A successful embodiment of such a thin cell separator comprises a plastic flow through mesh and an associated gasket which extends around the periphery of the mesh and is fabricated from a suitable polymer such as silicone rubber. One technique for constructing thin cell separators of this type includes filling the pores of the plastic mesh with a curable plastic by hand-squeezing a polymer material such as silicone rubber from a tube onto the plastic mesh in the desired configuration for the gasket. The mesh is then placed between polyethylene sheets and, through the use of a roller, the rubber is pressed down into a thickness approximately equal to that of the mesh. After curing, the holes and the outside edges of the separator are cut using a template, and the inside edges of the gasket "picked" to form a smooth edge. The drawback with this method is that while it produces quality separators, the time and labor required render the method impractical for commercial use.
Other methods such as injection molding and compression molding have also been used, but these suffer disadvantages such as the complexity, and hence expense, of the mold required, and difficulties regarding the maintenance of proper gasket thicknesses.
As explained in detail hereinafter, the present invention concerns a screen printing technique for forming separators. So-called "silk screening" itself is, of course, an extremely common, well known process. Further, British Pat. No. 1,212,839 concerns the use of screen printing in producing face-to-face seals or gaskets wherein a polymer is forced through a temporary screen into a pattern in a relatively thick stencil, the screen being removed after the gasket is formed. As will become clear from the discussion below, the method of the invention is quite different from that disclosed in the British patent. Other patents of possible interest include U.S. Pat. Nos. 2,460,168 (Caserta) 3,580,841 (Cadotte et al) and 3,696,742 (Parts et al.) although this listing is not, nor is it intended to be, exhaustive.